Fluorine-containing polymers, or fluoropolymers, are an important class of polymers and include for example, amorphous fluorocarbon elastomers and semi-crystalline fluorocarbon plastics. Within this class are polymers of high thermal stability and usefulness at high temperatures, and extreme toughness and flexibility at very low temperatures. Many of these polymers are almost totally insoluble in a wide variety of organic solvents, and are chemically inert. Some have extremely low dielectric loss and high dielectric-strength, and most have unique nonadhesive and low-friction properties. See, for example, F. W. Billmeyer, Textbook of Polymer Science, 3rd ed., pp. 398-403, John Wiley & Sons, New York (1984).
Amorphous fluorocarbon elastomers, particularly the copolymers of vinylidene fluoride with other ethylenically unsaturated halogenated monomers, such as hexafluoropropene, have particular utility in high temperature applications, such as seals, gaskets, and linings--see, for example, Brullo, R. A., "Fluoroelastomer Rubber for Automotive Applications," Automotive Elastomer & Design, June 1985, "Fluoroelastomer Seal Up Automotive Future," Materials Engineering, October 1988, and "Fluorinated Elastomers," Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 8, pp. 500-515 (3rd ed., John Wiley & Sons, 1979).
Semi-crystalline fluoroplastics, particularly polychlorotrifluoroethylene, polytetrafluoroethylene, copolymers of tetrafluoroethylene and hexafluoropropylene, and poly(vinylidene fluoride), have numerous electrical, mechanical, and chemical applications. Fluoroplastics are useful, for example, in wire coatings, electrical components, seals, solid and lined pipes, and pyroelectric detectors. See, for example, "Organic Fluorine Compounds," Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 11, pp. 20, 21, 32, 33, 40, 41, 48, 50, 52, 62, 70, 71, John Wiley & Sons, (1980).
Fluorine-containing polymers can be prepared by free-radical initiated polymerization of one or more fluorine-containing ethylenically unsaturated monomers. Free radicals are typically formed by the decomposition of a free-radical initiator. Free-radical initiators may be decomposed by light, heat, high energy radiation, or as a result of oxidation-reduction reactions. When free radicals are generated in the presence of free-radical polymerizable ethylenically unsaturated monomers, a chain reaction occurs producing polymer. The polymer can be prepared by polymerization of monomers in bulk, in solution, in emulsion, or in suspension. Fluoroelastomers and fluoroplastics are preferably prepared by aqueous emulsion or suspension polymerization because of the rapid and nearly complete conversion of monomers, easy removal of the heat of polymerization and ready isolation of the polymer. Emulsion or suspension polymerization typically involves polymerizing monomers in an aqueous medium in the presence of an inorganic free-radical initiator system, and surfactant or suspending agent.
Copolymers of tetrafluoroethylene ("TFE") and propylene, and terpolymers of TFE, propylene, and vinylidene fluoride are known and useful polymers. See, e.g., D. E. Hull et al., "New Elastomers are More Resistant to Many Automotive Fluids," SAE Technical Paper Series, #890361, SAE Publications Division, Warrendale, Pa., (1989), D. E. Hull et al., "New Type Fluoroelastomers With Improved Chemical Resistance to Automotive Oils and Lubricants," SAE Technical Paper Series, #900121, SAE Publications Division, Warrendale, Pa., (1989), Grootaert et al., "Elastomers, Synthetic Fluorocarbon Elastomers," Kirk-Othmer, Encyclopedia of Chemical Technology, Fourth Ed., Vol. 8, pp. 990-1005, John Wiley & Sons, (1993), Grootaert et al., "A Novel Fluorocarbon Elastomer For High-Temperature Sealing Applications In Aggressive Motor-Oil Environments," Rubber Chemistry and Technology, Volume 63, pp. 516-522, American Chemical Society (1990), and Kolb et al., "Aging Behavior of Fluorocarbon in Various Motor Oils," Automotive Polymers & Design, Volume 7 (No. 6), pp. 10-13, Lippincott & Peto, Inc. (1988). However, their manufacture has been known to be difficult, particularly with respect to the preparation of amorphous polymers derived from TFE and propylene. Various patents describe processes to make these polymers.
U.S. Pat. No. 3,859,259 (Harrel et al.) prepares certain amorphous copolymers of TFE and propylene by a continuous aqueous emulsion polymerization process at high pressure (preferably about 500 to 1,500 psig) using ammonium persulfate as initiator and sodium lauryl sulfate as the emulsifier.
U.S. Pat. No. 5,037,921 (Carlson) prepares certain fluoroelastomer copolymers of TFE and propylene by a semi batch, emulsion polymerization process in the presence of diiodo chain transfer agents. The polymerizations are preferably run at temperatures of 70.degree. C. to 90.degree. C. and preferably at pressures of 2.6 to 2.7 MPa (380 to 400 psig).
U.S. Pat. No. 3,933,773 (Foerster) prepares certain thermoplastic elastomeric copolymers of TFE and propylene by an emulsion polymerization reaction utilizing a redox initiator system at a pressure of 100 to 1,000 psig, preferably 250 to 350 p.s.i.g.
It is generally believed that one important problem in these polymerizations is degradative chain transfer reactivity of alpha-olefins containing an allylic hydrogen, e.g., propylene. See, e.g., Encyclopedia of Polymer Science and Engineering, Volume 13, pp. 714-715, John Wiley & Sons (1988), and George Odian, Principles of Polymerization, 2nd Ed., pp. 250-251, John Wiley & Sons. This degradative chain transfer is thought to be due to the weakness of the allylic carbon-hydrogen bond. For example, in propylene polymerizations, it is thought that a propylene molecule reacts with a propagating polymer-chain radical through transfer of its allylic hydrogen instead of through its double bond thus leading to low polymerization rates and resulting in polymers with low molecular weight. The formed allyl radical is resonance stabilized and unable to initiate a new polymerization. ##STR1## This reaction is also believed to be temperature dependent, and the polymerization rate is expected to decrease at higher temperatures. Therefore a great deal of effort has been put into development of low temperature redox initiating systems that would allow fast reaction rates and high molecular weight copolymers. Note that other monomers such as methyl methacrylate and methacrylonitrile, which also contain allylic carbon-hydrogen bonds, do not under go extensive degradative chain transfer because the ester or nitrile substituents are believed to stabilize the propagating radicals and decrease their reactivity toward transfer compared to olefins.
U.S. Pat. No. 4,277,586 (Ukihashi et al.) discloses a method for the low temperature (0.degree.-50.degree. C.) polymerization of TFE and propylene. The patent states in Col. 1 that "propylene-tetrafluoroethylene copolymers prepared by the conventional processes are characterized by low molecular weight . . . " In the method of the '586 patent "When the reaction temperature is above 50 .degree. C., the molecular weight of the copolymer will be decreased and the Mooney viscosity of the copolymer will be increased." (Col. 3, line 68, and Col. 4, lines 1-3). See also, G. Kojima and M. Hisasue, "Die Emulsionscopolymerisation yon Tetrafluoroethylen mit Propylen bei niedrigen Temperaturen," Makromol. Chem., Vol. 182, pp. 1429-1439 (1981).
In U.S. Pat. No. 4,463,144 (Kojima et al. ) this process was improved by means of an initiating system comprising a water soluble persulfate, a water soluble iron salt, a hydroxymethanesulfinate, and ethylenediaminetetraacetic acid or a salt thereof, in an alkaline aqueous solution containing a specific amount of tertiary butanol and an emulsifier at pH of up to 10.5. The tertiary butanol is said to act as an accelerator and "If the amount of tertiary butanol is less than 5 wt. %, no adequate effects are obtainable." (Col. 4, lines 5-7).
U.S. Pat. No. 5,285,002 (Grootaert) discloses the preparation of fluorine-containing polymers by polymerizing an aqueous emulsion or suspension of a polymerizable mixture comprising fluoroaliphatic-group containing sulfinate.